The invention relates to the field of plasma jet spectrophotometry, and more particularly to an improved sample introduction tube including a heat shield and radiant heat insulation.
Various plasma jet devices have been developed to generate a plasma jet for spectrometric analysis, or for studies of high-temperature chemical and physical phenomena of various materials. Generally, a plasma jet spectrophotometer includes a plasma jet device having a reaction or excitation zone which is stabilized in position. The reaction or excitation zone is obtained from the employment of a pair of electrodes of one polarity in combination with a third electrode of a different and opposite polarity than the first pair of electrodes. Generally, the electrodes in the first pair of electrodes are spaced apart in a position such that their axes, if extended, would intersect at an angle of anywhere from 60 to 90 degrees, while the third electrode is spaced apart from the angle of intersection of the first pair of electrodes and is offset from the plane formed by the intersecting axes, and typically and preferably offset substantially at right angles from the plane so as to form in operation a plasma jet of a column of ionized gas between electrodes, with the plasma jet being characterized by an inverted Y-form. The excitation or reaction zone in the Y-shaped plasma jet is formed at the lower region of the intersection of the extended axis of the first pair of electrodes. Such a plasma jet device is described in U.S. Pat. No. 4,147,957 issued on Apr. 3, 1979. As described in the referenced patent, directly below the intersecting angles of the first two electrodes, there is usually disposed a nebulizer or aerosol sample introduction tube having an outlet through which a sample of material can be placed in aerosol form in a gas stream. The device includes two constant current dc power supplies which provide power to the three electrodes of the device. In operation laminar flow of an ionized gas is maintained around the three electrodes. A sample material is then introduced as a laminar flow of an aerosol in an argon carrier through the nebulizer or aerosol sample introduction tube, and the outlet directly into the excitation zone formed by the plasma jet of the device. In a plasma jet the heated gas may reach a temperature in the region of 50,000 degrees Farenheit. Generally, the plasma jet at that temperature transfers heat to the sample introduction tube by three processes:
(1) Conduction, i.e. the transfer of heat from one part of the body to another part or to another body by short-range interaction of molecules and/or electrons;
(2) Convection, i.e. the transfer of heat by the combined mechanisms of fluid mixing and conduction; and
(3) Radiation, i.e. the transmission of energy in the form of electromagnetic waves.
Radiation incident on the sample introduction tube is generally absorbed resulting in high temperatures, and as can be appreciated from the very high temperature of the plasma jet, a substantial amount of the heat transfer to the sample introduction tube is in the form of radiant-heat transfer.
One of the disadvantages of the conventional plasma jet assembly is that the sample introduction tube is disposed in close proximity to the excitation zone. Under normal conditions, when only inorganic solids are dissolved in a liquid phase, the solids are not greatly affected by the radiant, convective and conductive heat transfer to the sample introduction tube. However, if the dissolved solids are organic in character, for example sugar, there is a tendency for solids to decompose (e.g. the sugar will caramelize) and form globules just inside the tip of the sample introduction tube. When this happens, the uniform laminar flow necessary for the proper operation of the system will be disturbed thereby deviating the path of the sample stream and in turn, altering the results.
In U.S. Pat. No. 4,080,550 a plasma generator having a cooled annular cathode is disclosed. In the device a plurality of cooling passages surrounding the outlet orifice disposed within the annular cathode are disclosed. The passages are designed to maintain the temperature of the outlet orifice below the agglomeration temperature of the solids and the solids containing fluid medium being injected through the linear feed channels of the device. The cooling passages provide extensive cooling of the orifice outlets. Although this type of cooling arrangement is practical in conjunction with a conical cathode, such a cooling unit cannot be efficiently used with the plasma jet device and described in U.S. Pat. No. 4,147,957. There, the anodes are not conical but rather rectilinear and disposed in close proximity to sample introduction tube.
Numerous other plasma generation devices have been disclosed in the art, for example, U.S. Pat. No. 3,614,376 (Manabe, et al.); U.S. Pat. No. 3,818,174 (Camacho); and U.S. Pat. No. 3,858,072 (Dembovsky). These devices include some description of electrical insulation of the electrodes, and ways of stabilizing the flow of the gas stream, but do not describe an adequate means of insulating the sample introduction tube from the radiant and other heat transfer from the reaction zone.
It is accordingly an object of the present invention to provide a means for insulating the sample introduction tube from radiative, convective and conductive heat transfer from the plasma reaction zone.
It is another object of the present invention to provide a means for preventing agglomeration of organic matter in a sample introduction tube of a plasma jet in a spectrophotometer.
It is another object of the present invention to accomplish the aforementioned object without substantial or permanent modification to the equipment and without additional expensive equipment.